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This uses the built-in atomic functions. The test program is:

#include "XrdSys/XrdSysAtomics.hh"

int a;

void val_set(int v) {  AtomicCAS(a, a, v); }
int val_get() { return AtomicGet(a); }

The assembler code for x86_64 is as expected:

_Z7val_seti:
        .cfi_startproc
        movl    %edi, a(%rip)
        ret
        .cfi_endproc

_Z7val_getv:
        .cfi_startproc
        movl    a(%rip), %eax
        ret
        .cfi_endproc

While for ARM it looks like this:

_Z7val_seti:
        .fnstart
.LFB0:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        ldr     r3, .L2
        str     r0, [r3, #0]
        bx      lr
.L3:
        .align  2
.L2:
        .word   .LANCHOR0
        .cantunwind
        .fnend
_Z7val_getv:
        .fnstart
.LFB1:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        ldr     r3, .L5
        ldr     r0, [r3, #0]
        bx      lr
.L6:
        .align  2
.L5:
        .word   .LANCHOR0
        .cantunwind
        .fnend

So, the conclusion is that for x86_64 there is no difference whether we use or not __sync_fetch*. Obviously this also works without c++11. Let me know which one you prefer between this one and #265.

You can view, comment on, or merge this pull request online at:

  https://github.com/xrootd/xrootd/pull/266

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